1. Field of the Invention
This invention relates to improvements in and concerning a composition for a reduction-reoxidation type semiconductive ceramic capacitor produced by forming an insulating layer on the outer periphery of a semiconductive ceramic article by reoxidation.
Compact, high quality components for electronic equipment are in strong demand. This is also true of reduction-reoxidation type semiconductive ceramic capacitors, which have come to require high insulation resistance and breakdown voltage, small size, large capacity and small dielectric loss.
2. Prior Art Statement
Reduction-reoxidation type semiconductive ceramic capacitors are the subject of numerous patent publications and technical papers. Of such capacitors, those which have been practically applied are such that when the temperature characteristic of electrostatic capacity is fixed in the range of +20 to +30%, the dielectric constants thereof are so low as to fall in the range of 3,000 to 5,000. Japanese Patent Public Disclosure SHO No. 53(1978)-114100 discloses a ceramic material having a Ni compound incorporated in a BaTiO.sub.3 -BaZrO.sub.3 -Bi.sub.2 O.sub.3 -TiO.sub.2 type solid solution. The ceramic capacitor composed of this material exhibits such characteristic properties as 0.57 .mu.F/cm.sup.2, tan .delta.4.3%, an insulation resistance of 200 M.OMEGA./cm.sup.2 (15 VDC, after 15 seconds' standing), and a temperature characteristic (%) of electrostatic capacity at -30.degree. to 85.degree. C. within .+-.30%. Japanese Patent Public Disclosure SHO No. 62(1987)-30481 discloses a ceramic material having a Ni compound incorporated in a BaTiO.sub.3 -BaZrO.sub.3 -Bi.sub.2 O.sub.3 -TiO.sub.2 type solid solution. The ceramic capacitor composed of this material exhibits such characteristic properties as 0.60 .mu.F/cm.sup.2, tan .delta. 4.5%, an insulation resistance of 210 M.OMEGA./cm.sup.2 (15 VDC after 15 seconds' standing), and a temperature characteristic (%) of electrostatic capacity at -30.degree. to 85.degree. C. within .+-.30%.
Reduction-reoxidation type semiconductive ceramic capacitors with even better properties are, however, desired. In the industry, therefore, there is a desire to develop a ceramic composition which has, as essential requirements, a breakdown voltage of not less than 400 V and a temperature characteristic within .+-.30% and possessing the characteristics of a large capacity per unit area and a low dielectric loss. Various studies are now under way in search for such a composition. In the reduction-reoxidation type semiconductive ceramic capacitor, generally the thin dielectric layer in the surface region has a very great influence on the capacity, the temperature characteristic of the capacity, the insulation resistance, the breakdown voltage, and the dielectric loss. The capacity per unit surface area increases and the insulation resistance and the breakdown voltage decrease when the thickness of the dielectric layer decreases. For increasing the insulation resistance and the breakdown voltage, it is necessary to increase the thickness of the dielectric layer and as a result, the capacity per unit surface area decreases. The dielectric loss of the capacitor depends on the magnitude tan .delta., of the dielectric layer. The DC resistance in the boundary between the dielectric layer and the semiconductive ceramic article contributes more to the dielectric loss. For the magnitude of tan .delta. to be decreased, this DC resistance must be decreased. Particularly, since the magnitude tan .delta. constitutes an important characteristic for the capacitor, an increase in this magnitude forms a serious drawback. The largest possible decrease of this magnitude is the most significant task in the improvement of various properties. Success in this task requires the development of a novel ceramic composition whose insulation resistance and breakdown voltage per unit thickness of dielectric layer are high, whose DC resistance is low and whose capacity is high. To be specific, the composition must possess characteristic properties of not less than 400 V of breakdown voltage, not less than 10.sup.9 .OMEGA. of insulation resistance, not less than 0.65 .mu.F/cm.sup.2 of capacity per unit surface area and less than 2.5% of tan .delta..